The present invention relates to optical gap measuring tool calibration. More specifically, the invention relates to a system for calibrating a hard disc drive magnetic head flying height tester by optical interference techniques.
FIG. 1 provides an illustration of a typical hard disc drive. In the art of hard disc drives, magnetic read/write heads 102 are commonly integrated in a slider 102 designed to respond to a flow of air moving with the rotating disc 104 over which the slider 102 travels. The head/slider 102 ‘flies’ close to the surface of the disc 104. In manufacturing such heads/sliders 102, it is often necessary to test hydrodynamic characteristics of the heads 102 to verify their performance. It is important that the head 102 not travel too far from or close to the disc 104 surface. Further, it is important to prevent the head 102 from traveling at an improper angle with respect to the disc surface 104. A head 102 traveling too high above the disc surface 104 will result in a lower than desired areal density. A head 102 traveling too low can cause an interface failure between the head 102 and disc 104.
In order to test the flying height of the head, a flying height tester is commonly used. Optical interference techniques are employed to determine the distance between head and disc. A monochromatic light source is directed at a transparent surrogate disc, such as a glass disc, rotating at speeds similar to that of a magnetic disc, and the head assembly being tested is secured in a holder in its normal flying orientation in relation to the disc. The monochromatic light is directed at the disc at a predetermined angle to the surface thereof. The light is reflected from the surface of the disc closest to the head, as well as from the surface of the flying head itself, and impinges onto a light sensitive sensor.
The interference effects created by the combined reflections from the disc and the slider surface provide the flying height information. A computer receives data from the flying height tester and calculates the perceived flying height and angle of the head. As hard drives become smaller and increase in data storage capacity, the desired head flying height continually reduces. Therefore, the accuracy of a flying height tester, and thus its calibration, are of critical concern.
In the art, calibration of flying height testers has been accomplished through the use of a standard head whose characteristics are known. However, after repeated use, the reflective surface and flying characteristics of the head are altered by dust, oil and other foreign matter. These contaminants can alter the standard for calibration purposes. The calibration of flying height testers is also performed by a standard comprising a substrate having a reflective layer deposited thereon to represent the head and a transparent layer having a predetermined thickness deposited on the reflective layer. The standard is placed in the flying height tester with the transparent layer spaced from the disc and monochromatic light directed at the standard. A disadvantage of such a standard is that it uses a transparent material rather than air between the disc and the reflective layer. In addition, such a standard does not provide for the accurate determination of position along the length of the standard.
FIGS. 2a–b illustrate another flying height tester (optical gap measuring tool) calibration standard known in the art and described in U.S. Pat. No. 5,453,831 to Li et al. A wedge slider 34 is held in contact with a transparent disc 32. One end 42 of the wedge slider is raised, creating an optical wedge between the wedge slider 34 and the disc 32. The wedge slider 34 has a first rail 52 and a second rail 54, each of which extends along the length of the wedge slider 34 and has a surface 55 facing the disc 32. The first rail 52 has a plurality of cylindrical portions 56 therein at regularly spaced intervals. Each cylindrical portion 56 has a diameter equal to the diameter of the beam spot from the light source 72 of the flying height tester, thus allowing the beam spot to be matched at the position of any given cylindrical portion 56. The second rail 54 has a width that is greater than the diameter of the beam spot, thereby allowing for a continuous optical wedge measurement along its length. The second rail 54 may also have a plurality of marks on one side, which can be used to determine the position along the length of the wedge slider at which a measurement is taken.
To calibrate the flying height tester, the distance between the wedge slider 34 and the disc 32 is measured at multiple locations along its length and compared with known, or expected, values of the flying height at those locations. The flying height is measured at multiple locations by optical interference techniques. The expected value of the flying height at each position along the length of the wedge slider is calculated using the known dimensions of the wedge slider. The calculation is corrected for any surface irregularities found during a mapping of the surface of the first and second rails.
One disadvantage of a flying height tester calibration standard such as this is the complexity of design. Manufacturing such a device at such a small scale is very difficult and expensive. Further, the likelihood of form and material irregularities increases with complexity of design. Still further, the determination of the exact lateral position of the measurement is a problem, which is only partially resolved by the incorporation of the cylindrical portions 56 in the design described in FIG. 2b (causing increased design complexity).
Yet another disadvantage of this design is the effect of phase change upon reflection. This difficulty is described in an article entitled “Interferometric Measurement of Disk/Slider Spacing: The Effect of Phase Shift on Reflection” by C. Lacey, T. Shelor, A. J. Cormier, and R. E. Talke. This article provides that the optical properties of slider materials can introduce errors as large as 20 nanometers (nm) in flying height sensors. These same problems apply to calibration standards. To compensate for such potential errors, the calibration standard must itself be carefully set up for phase change on reflection, using ellipsometric techniques. For example, U.S. Pat. No. 5,453,831 recommends fabricating a separate block of the same material as that of the wedge slider calibration standard. This piece of material is assumed to have the same optical properties as the wedge slider and may be used to determine the phase change on reflection using an ellipsometer or like optical instrument. This separate step complicates the current process further, as well as introducing uncertainties into the calibration.
FIG. 3 illustrates another calibration standard known in the art and described in U.S. Pat. No. 5,724,134 to de Groot et al. As shown in FIG. 3, the apparatus is comprised principally of two elements 10,20. One or both of the elements is fabricated from a substantially transparent material such as glass or the like, thus permitting access to the gap for optical inspection. One of the surfaces of the first of these elements is substantially flat or planar 15, and one of the surfaces of the second element is non-planar or curved 25. The curved surface 25 is preferably convex spherical in form. The two principle elements of the apparatus of the present invention are held together in such a way that the curved surface of the second element is substantially in contact with the flat surface of the first element. Since a curved surface and a flat surface cannot be entirely in contact over the entire area of either one of the two surfaces, the region of contact 80 is typically substantially smaller in area than either of the two surfaces. Outside the region of contact 80, the gap between the surfaces varies according to the known geometric curvature of the surface on the second element.
One problem with this design involves the measurement uncertainty within the contact area (zeroed region). Between the first element 10 and second element 20 there is a contact area 80 of at least 3 millimeters (mm). This is due to physical deformation towards the center of the curved surface (caused by the pressing of the first element 10 to the second element 20). The broad region of contact makes it difficult to resolve accurate height information with respect to measurement location for calibration. The amount of physical deformation affects the resulting calibration data.
FIGS. 4a–b illustrate an example of the adverse effects of surface deformation occurring in a calibration standard such as is described in the '134 patent. As shown in FIG. 4a, with minimal pressure and slight deformation, the calibration standard provides a zeroed reading at the center of the curved surface (z=0 nanometers(nm) at x=6 millimeters(mm)) 402. By comparison, with greater deformation of the center surface, it is more difficult to resolve the contact location. The chart actually shows a negative height value between 4.5 and 7.5 (See reference 404). Once the first element 10 is affixed to the second element 20 (typically by adhesive), the deformation is fixed as well. With this deformation when measuring such small gaps, it is very difficult to determine where the contact point is.
It is therefore desirable to have a system for calibrating flying height testers that avoids the above-mentioned problems, as well as having additional benefits.